[XFRM] xfrm_user: Better validation of user templates.

Since we never checked the ->family value of templates
before, many applications simply leave it at zero.
Detect this and fix it up to be the pol->family value.

Also, do not clobber xp->family while reading in templates,
that is not necessary.

Signed-off-by: David S. Miller <davem@davemloft.net>